The invention relates to an apparatus for the continuous welding or sealing of seams of plastic films, which includes supply rolls for flat films and for at least one sealing strip.
Continuously operating film-welding or sealing machines use continuous heated steel bands. They are employed in packaging technology, for example, for sealing head seams on filled plastic pouches or bags in a continuous operation.
In a welding and sealing machine known in the state of the art, two endless steel bands are guided respectively over two deflecting or tensioning rollers. The bands are arranged in such a way that they touch one another and overlap over their entire surface in one of the tensioned sections between two tensioning rollers. The bands are driven in the same direction by means of the rollers and convey the films located between them.
For heating the film to the sealing temperature and for exerting a sealing pressure, fixed, continuously heated heating elements slide on the rear sides of the conveying steel-band sections in the conveying direction. As a result of thermal conduction via the steel bands, the sealing heat is transmitted to the films to be sealed or welded to one another.
Sealing and welding can be matched to a particular film by varying the temperature and the pressing force of the heating elements, the conveying speed of the steel bands and the length of the heating zone.
However, serious disadvantages arise because of the high sliding friction between the heating elements and the steel bands. The sliding friction leads to wear and causes abrasion. Additionally, higher sealing speed cannot be maintained in continuous operation. Another disadvantage is uneven specific distribution of the sealing pressure. This is a result of the warping of the heat-transmitting sliding faces, which occurs because of thermal expansion in the heating elements, which may, for example, be in the form of heating jaws. This also results in an uneven temperature distribution in the individual steel bands because of differing heat transfer resistances between the heating jaws and the steel band.
On the whole, the known welding and sealing machines are not suitable for the reliable production of high-standard welding or sealing seams at relatively high sealing speeds in continuous operation.
Published European Patent Application No. 0 080 120 makes known an apparatus for producing a tube from a flat film web, at the same time forming a longitudinal seam. This apparatus comprises a forming body with an inner bore of essentially circular cross-section, for supporting the formed tube externally in the region of the mutual overlapping of the marginal zones of the web; and a circular-cylindrical inner mandrel for supporting the formed tube internally. Furthermore, for guiding and spacing the web edges of the film web, there is a flexible strip which has a bent cross-section and a curvature which can be matched to the tube periphery, with one or two projecting borders on the longitudinal edges. The tubes produced by means of this apparatus are generally sealed on a welding and sealing machine operating intermittently, such as is described above as being known in the art. Because of the intermittent sealing, the working speed of sealing machines of this type is low, and is approximately 10 m of tube per minute.
To produce a tube from a flat film web, folding aids, such as forming shoulders or forming heads, as described in German Offenlegungsschrift No. 3,013,320, are conventionally used. These bend and fold the web so that they form a longitudinal seam, along which bonding is effected. The two marginal zones can also be connected to one another by means of a film strip--a so-called sealing strip--and it is sufficient for the marginal zones to butt against one another without overlapping.
U.S. Pat. No. 2,148,884 describes an apparatus which uses for support a mandrel arranged inside the formed tube, while a roller is pressed against the tube from outside. These folding aids do not guarantee exact guidance of the overlapping marginal zones of the web, so that the tube formed does not have a uniform periphery.
The forming device known from U.S. Pat. No. 2,856,742 has two forming parts which are located opposite one another and which each come arcuately up against one marginal zone of the web from the outside. The two forming parts are arranged at a distance from one another in the direction of the tubular web and are therefore unsuitable for those webs in which an adhesive is coated on one marginal zone before the forming of the tube. The overlapping marginal zones are only separated from one another in the region between the two forming parts, so that there is danger that the marginal zones will bond to one another even before they pass the first forming part.
U.S. Pat. No. 2,285,263 describes a device for tube formation in which the web is supported from inside and outside and is guided at its margins. The web is supported externally by a curved steel body which brings the web margins towards one another in a helical fashion. The web material is subjected to severe mechanical stress here, since during the formation of the tube the web is forced by the metal body to assume a predetermined form even before the overlapping of its margins.